The present invention relates to a reactive adsorption process for separating N2O from a mixture that includes other nitrogen oxides (xe2x80x9cNOxxe2x80x9d) including NO and NO2/N2O4 and, in particular to a process for selectively converting NO in a mixture by an oxidant to NO2/N2O4, which is selectively adsorbed from the mixture by an adsorbent and recovering a substantially reduced NOx stream of N2O stream. The adsorbed NOx can be recovered from the adsorbent by a number of procedures such as elevating the temperature, reducing the pressure, inert gas purging, water washing or any combination thereof.
Nitric acid oxidation of hydrocarbons is a well known oxidation reaction. This reaction has been applied in chemical industry to produce important chemicals and chemical intermediates such as adipic acid for many years. Unfortunately, this reaction can produce a significant amount of N2O by-product (xcx9c1 mole of N2O produced per mole of adipic acid produced). NO and NO2/N2O4 is also produced in this nitric acid oxidation reaction. The co-existence of NO and NO2/N2O4 with N2O prevents the off-gas from being used in other medical or industrial applications. Furthermore, all of these nitrogen oxides have a significant negative impact on the environment. Presently, all nitric acid oxidation reaction users are required to spend significant amounts of money as capital investment and operating costs to abate their off-gas before the off-gas can be released to the environment.
The status of various off-gas abatement processes in the adipic acid industry and potential new abatement technologies were reviewed by Reimer et al. xe2x80x9cAbatement of N2O Emissions Produced in the Adipic Acid Industryxe2x80x9d R. A. Reimer, et al, Environmental Progress, vol. 13, no.2 p.134 (May 1994). Tretijak et al., European Patent, EP 625369; Fetzer et al., German Patents DE 4301470, DE 4224881, DE 4128629 also described various processes and catalysts for decomposing nitrous oxide in nitric acid oxidation off gases by catalytic reactions. EPA technical report EPA-450/3-91-026 provides a review of various NOx control techniques for nitric acid and adipic acid processes. For use in nitric acid plants, extended absorption, non-selective catalytic reduction and selective catalytic reduction are used most often. For adipic acid plants extended absorption and thermal reduction are used most often.
Badische""s FR 1531266, described a process to selectively remove NO2 and O2 from a mixture of nitrogen oxides, and O2 by catalytic reduction with H2 using an Ag2Oxe2x80x94MnO2xe2x80x94Al2O3 catalyst. Using this process, only NO2 and O2 can be removed. NO still co-exists, with N2O; therefore, this stream needs further treatment. Nakai et al., JP 661021 describes a process for refining nitrous oxide from a mixture of other nitrogen oxides and CO2. This process consists of passing the mixture gas through an NaOH absorption tower first and then an KMnO4xe2x80x94NaOH absorption tower in order to remove NO and NO2 from the N2O stream. However, in this process, due to the interference from CO2, large amounts of NaOH are consumed to remove NOx as well as CO2. Furthermore, a liquid waste stream is generated which must be further treated.
The prior art does not recognize that an adsorbent can be used to selectively adsorb NO2/N2O4 from a mixture of NO, NO2/N2O4 and N2O. Nor does the art recognize that a reactive adsorption process can be used to first selectively convert NO in the mixture with an oxidant to NO2/N2O4 and then NO2/N2O4 can be selectively adsorbed from N2O in the mixture. Therefore separate NO2O from other nitrogen oxides in a mixture. By using this reactive adsorption process, both N2O and the remaining nitrogen oxides can be recovered as useful products and zero waste is generated.
Accordingly, it is an object of the present invention to provide a method for effectively removing a substantial portion of NO2/N2O4 from a mixture containing N2O in a gaseous mixture stream. It is also an object of the invention to provide a reactive adsorption process that can be used to first selectively convert NO in a mixture with an oxidant to produce NO2/N2O4 and then to selectively adsorb produced NO2/N2O4 from the N2O in the mixture stream, thus separating the N2O from other nitrogen oxides. A further object of the invention is to provide a N2O stream having a significantly reduced NOx that is suitable for use as a feedstock for other medical or industrial applications and to provide for the recovery of the adsorbed NOx which can be reconverted to nitric acid. It is also an object of the invention to provide a method which can reduce abatement costs and generate two useful products.
Generally, the present invention is a reactive adsorption process for separating NOx from N2O in a mixture. The process comprises (A) selectively converting with an oxidant NO in a mixture to NO2/N2O4; (B) selectively adsorbing NO2/N2O4 from the mixture by contacting same with an adsorbent; and (C) recovering a stream of N2O having a substantially reduced NOx concentration. The adsorbed NOx can be recovered from the adsorbent by elevated temperature, reduced pressure, inert gas purge, water wash or any combination thereof. The advantages of the invention will be better understood with reference to the following detailed description of presently preferred embodiments of the process taken in connection with illustrative examples.
In a preferred embodiment of the invention N2O is present as one of the major components of a mixture and other nitrogen oxides and an oxidant are present in minor amounts. The mixture may also comprise minor amounts of such other compounds as CO, CO2, N2, other inert gases or hydrocarbons.
A suitable oxidant for use in the present invention is preferably selected from the group consisting of O2, O3, H2O2 and KMnO4. An especially preferred oxidant is O2.
The oxidant concentration in the mixture is not critical and can generally be in the range of 0.01% to 40%. A preferred oxidant level is within a range of 0.5% to 21%.
The NO oxidation reaction can be conducted with or without the presence of a catalyst. However, it is preferable to utilize a catalyst for the NO oxidation reaction selected from the group comprising activated carbons, carbon molecular sieves, carbonaceous chars, zeolites, silica gel, activated alumina, manganese oxides, copper oxides, aluminum silicate, carbo alumina gel, iron oxide, nickel oxide, cobalt oxide, various platinum group metals and any combination of thereof. A preferred catalyst is an activated carbon.
The mixture comprising the nitrogen oxides, including oxidant and N2O can be contacted with the adsorbent by standard methods. A suitable adsorbent for this process is selected from the group comprising activated carbons, carbon molecular sieves, carbonaceous chars, zeolites, silica gel, activated alumina, silicates, clay(s), adsorbent polymers, and any combination thereof. A preferred adsorbent is an activated carbon.
The NO oxidation reaction and the NOx adsorption can be conducted in two separated steps or in one combined oxidation/adsorption step. The presently preferred embodiment of the process is to conduct the selective oxidation reaction and adsorption in a single step. A suitable material (an oxidation catalyst/adsorbent) for this combined oxidation/reaction step is selected from the group comprising an activated carbon, carbon molecular sieve, carbonaceous char, zeolite, silica gel, activated alumina, silicate, clay(s) or any combination of the above mentioned catalysts with adsorbents. A preferred material to be used in the combined oxidation/adsorption step is an activated carbon.
The process of the invention can be carried out as a batch process, a semi-batch process or a continuous process. In a preferred embodiment, the mixture is contacted with one or more packed-beds of the oxidation catalyst/adsorbent operated in a cyclic mode. For example, the mixture can be passed through a first packed-bed of oxidation catalyst/adsorbent until the oxidation catalyst/adsorbent is nearly saturated with NOx, the mixture can then be diverted to a second packed-bed of oxidation catalyst/adsorbent bed. The NOx can then be recovered from the nearly saturated bed by reducing the pressure, increasing the bed temperature or a combination of these two. A heated sweep gas such as air may be passed through the saturated bed to facilitate the desorption and recovery of the NOx. The mixture can then be switched between the oxidation catalyst/adsorbent beds allowing for the oxidation catalyst/adsorbent beds to alternate between a reactive adsorption mode and desorption mode, thereby producing a substantially continuous process.
The temperature at which the NO oxidation is conducted is not critical and can generally be less than 300xc2x0 C. A preferred temperature is within a range of 10xc2x0 C. to 80xc2x0 C. The pressure at which the NOx oxidation is conducted is not critical and can generally be within a range of 0.1 atm to 20 atm. A presently preferred pressure is within a range of 1 atm to 10 atm.
The temperature at which the mixture is contacted with the adsorbent is not critical and generally is less than 180xc2x0 C. A presently preferred temperature range is within 10xc2x0 C. to 80xc2x0 C. The pressure at which the mixture is contacted with the adsorbent is not critical and can generally be within a range of 0.1 atm to 20 atm. A presently preferred pressure range is within 1 atm to 10 atm.
The physical form of the adsorbent is also not critical to the present invention and can be, for example, flakes, chips, pellets or powder.
The reactive adsorption process of the invention selectively reacts and adsorbs nitrogen oxides including NO and NO2/NO2O4 relative to N2O in a mixture. This selectivity allows for recovery of N2O in reduced NOx concentration. Recovery of the N2O reduced in other nitrogen oxides can be effected by standard methods for separating gases from solids. In the preferred process using a packed-bed of oxidation catalyst/adsorbent, recovery of the N2O can consist of collecting the effluent from the column in a suitable container or recompressing the gas for use directly in down-stream processes.
The present process can further comprise recovery of the adsorbed nitrogen oxides by desorption from the adsorbent or the oxidation catalyst/adsorbent. Desorption of the adsorbent or the oxidation catalyst/adsorbent can be effected by standard methods such as temperature swing, pressure swing, inert gas purge, water wash or any combination thereof.
The following examples are provided to illustrate the present invention. These examples are not intended to limit the scope of the claim herein.